Reference half cells serve in electrochemical measuring probes, for example, in measuring probes of potentiometric sensors, to deliver a constant reference potential for measurements with one or a plurality of measuring half cells. They are frequently applied in many fields of application as rod-shaped reference electrodes or combined with a measuring electrode to form a so-called single-rod, measuring chain. Measuring electrodes, with which the reference electrodes are applied in combination, are, for example, pH glass electrodes, pH semiconductor electrodes or ion-selective electrodes for determining concentration of cations, such as sodium, potassium, calcium, or anions, such as chloride, fluoride, nitrate and carbonate, in a measured medium. Such electrode combinations serve, for example, for determining corresponding ion concentrations in aqueous solutions or water-containing, measured media, such as natural waters, swimming pools, waste waters or product streams.
It is known, that that part of the reference electrode or reference half cell, which is brought in contact with a measured medium in performing the determining must assure an electrolytic contact of the reference electrolyte located in the reference electrode with the measured medium. The terminology, “electrolytic contact”, means here a liquid contact between the reference electrolyte of the reference electrode and the measured medium, such that an exchange of ionic charge carriers is permitted. Such a contact can be accomplished by a diaphragm arranged in the housing wall of the reference half cell for permitting diffusion both from the reference electrolyte into the measured medium as well as also in the opposite direction. The term, “diaphragm”, refers here to any contact location allowing such a diffusion. This can be formed, for example, by a ceramic, synthetic material or plastic body, a plug of crosslinked hydrogel, a slit, one or a number of bores, or a ground glass.
Reference electrodes comprise, typically, a reference electrolyte accommodated in a reference electrolyte chamber. Such is frequently an aqueous KCl solution, into which extends a potential indicating element coated with a difficulty soluble salt of the reference electrolyte, e.g. a silver wire coated with AgCl. In the case of such reference electrodes, basically, there is the problem that they do not really have a constant potential, this being due to diffusion processes at the diaphragm, i.e. through diffusion of the reference electrolyte accommodated in the reference electrolyte chamber into the measured medium or from diffusion of the measured medium into the reference electrolyte. Furthermore, also a plugging of the diaphragm is possible, for example, through the crystallizing of difficulty soluble salts. This likewise influences the stability of the potential of the reference electrode.
An opportunity for improving the stability of the potential is to supply the reference half cell with a positive pressure relative to the environment of the reference half cell housing. Reference electrolyte then flows through the diaphragm into the environment, e.g. into the measured medium, into which the reference half cell extends during measurement operation. In this way, on the one hand, continuously very small amounts of reference electrolyte enter into the measured medium, while, on the other hand, the reference half cell remains in proper condition and the diaphragm is cleaned and internally rinsed by the continuous flow of electrolyte solution. Thus, clogging of the diaphragm or a poisoning of the reference electrolyte by substances diffusing from the measured medium into the interior of the reference electrolyte chamber is effectively prevented or at least markedly delayed.
The positive pressure can result from the hydrostatic pressure of the liquid column of the reference electrolyte within the reference electrode. Since the reference electrolyte continuously escapes from the reference electrode, correspondingly, either a relatively large liquid electrolyte reservoir is required within the reference electrode or else there must be a regular refilling of reference electrolyte solution into the reference half cell.
In DE 20 2006 017 215 U1, measuring probes with glass electrodes for pH measuring are described, which have a refillable electrolyte reservoir, also referenced as a paunch or bladder, for accommodating liquid electrolyte. The electrolyte reservoir has an opening, through which a pressurizing medium can enter into the electrolyte reservoir, so that a continual flow of the electrolyte from a diaphragm is assured, in order to avoid penetration of measured medium into the electrode interior. As described in DE 20 2006 017 215 U1, such electrodes require special assemblies, by means of which they can be connected to a process container. An opportunity, in situ, i.e. during measurement operation, to replenish electrolyte into the electrolyte reservoir, is not provided.
In DE 20 2005 009 297 U1, a coupling apparatus for a reference electrolyte supply from an external supply to a reference media container of a potentiometric sensor is described. The coupling apparatus includes a first supply side, coupling body and a second sensor side, coupling body connectable releasably mechanically therewith. Each of the coupling bodies has a reference electrolyte duct, wherein the reference electrolyte duct of the second coupling body is sealed by means of an elastic sealing element. The first coupling body has a hollow needle, which, when the coupling bodies are mechanically connected with one another, pierces through the sealing element, in order to assure a flow connection between the two reference electrolyte ducts of the two coupling bodies sealed relative to the environment. If the coupling bodies are separated from one another, the sealing element should reduce leakage of reference electrolyte from the potentiometric sensor.
The handling of the coupling apparatus described in DE 20 2005 009 297 U1 is, indeed, simple and little susceptible to error. However, in the connecting and separating of the coupling bodies, a leakage of electrolyte from the supply side coupling element is not suppressed, so that there is the danger of a contamination of the environment, or a service person, by escaping reference electrolyte. On the whole, construction of the coupling apparatus is relatively complicated, which correspondingly increases the piece costs of the individual potentiometric sensors.
In DE 39 40 948 A1, a method for continuous electrolyte supply to a reference electrode system and a corresponding reference electrode system are described. In such case, an electrode insert held sealed in an outer seating tube and removable from the seating tube is provided, wherein, by the removal motion, an inner valve mechanism is closed, through which otherwise the liquid, reference electrolyte is fed via the outer seating tube to the electrode insert. In the case of the reversed introductory movement of the electrode insert, the valve mechanism is reopened. The structure of the valve mechanism includes a tube insert connected with the outer seating tube and provided with an electrolyte supply duct, relative to which a rotary sleeve coupled with the electrode insert via a driving pin is rotatable upon removal, or introduction, of the electrode insert. Through the twisting of a transverse bore in the rotary sleeve relative to the electrolyte supply duct in the tube insert upon the removal, or introductory, movement, a connection between the electrolyte supply duct and the interior of the electrode insert is interrupted, or produced, as the case may be.
The described mechanism is very complicated and complex to manufacture. Furthermore, in the case of removal of the electrode insert, only the electrolyte supply relative to the environment is sealed; the opening of the electrode insert, via which electrolyte is fed into the electrode interior, remains, however, open. Thus, also in the case of this construction, the risk of leakage is not excluded in satisfactory manner.